The invention relates to a method and combination of devices for providing the comparability of spectrometer measurements, preferably in the near infrared range, with a plurality of measuring instrument individuals of the same type in a family.
NIR spectrometers have gained wide application. They are used to analyze gaseous and solid substances. With constantly increasing demands for accuracy, exact calibration of these instruments gains particular significance. It is especially important that the measured values of a specimen obtained with one spectrometer be replicable on another spectrometer of the same type.
Moreover, the accuracy of the spectral measurements is impaired by changes in the specimen being measured itself, for instance with regard to its consistency or dimensions in the case of nonhomogeneous materials. A standardizing method for reducing particle size effects is described for instance in U.S. Pat. No. 5,132,538.
It is well known to use natural (absolute) standards for standardizing the measuring instruments, among them for instance using a spectral absorption band of a gas to calibrate the wavelength scale of a spectrometer. In the field of infrared and near infrared spectroscopy, the standardization of the wavelength scale in this way is state of the art. On the other hand, the scale for the intensity is not standardized. Specimens are measured relative to a "hundred percent" standard specimen or reference. In measurements of reflection, this typically means a "hundred percent" reflector. In measurements of transmission, this typically means the signal magnitude at which there is no specimen in the beam of light. In addition, the "zero signal" can be measured by physically preventing the beam of light from reaching the detector. The signal of the specimen can then be corrected by means of the offset signal measured.
It is assumed of the signals S.sub.s and S.sub.r already corrected with the measured offset signal, it is assumed that S.sub.s /S.sub.r is directly proportional to I.sub.s /I.sub.r ;
S.sub.s means the signal when the specimen is measured; PA1 S.sub.r means the signal when the reference is measured; PA1 I.sub.s means the true signal for the light intensity that is reflected by the specimen or transmitted through the specimen; PA1 I.sub.r is the true signal for the intensity of the light that is reflected or transmitted by the reference. PA1 storage in memory of set-point spectra of immobilized standard specimens in a spectrometer individual of the family; PA1 ascertaining a number of comparison spectra of the standard specimens with the spectrometer individual of the family; PA1 calculating parameters of an approximation for the deviation in the comparison spectrum from the set-point spectrum per wavelength base point on the standard specimens; PA1 storage in memory of the parameters per wavelength base point in the instrument; PA1 measurement of the spectrum of an unknown specimen; PA1 calculation of true values for each wavelength base point of a measured value of the spectrum of the unknown specimen, using an equation that is obtained from the approximation function; PA1 outputting of the corrected spectrum as an actual spectrum.
In fact, the design of spectrometers takes this circumstance particularly into effect by designing the detectors in such a way that they are operated in the linear range of their characteristic curve. As much as possible, the user must then make sure that I.sub.s and I.sub.r are measured under the same peripheral conditions. Many cases exist, however, in which spectrometers of the state of the art, operated in the way described above, fail in measuring an intrinsically correct signal for the reflection or transmission of a given specimen. The differences in spectral sensitivity (on the scale of intensity) between spectrometer individuals can be very great, even in those of the same family. In some cases, the sensitivity of spectrometers is directly proportional to I.sub.s /I.sub.r, but the linearity coefficients of the instruments differ from one another. In other cases, S.sub.s /S.sub.r is linked in a more-complex way to I.sub.s /I.sub.r, namely nonlinearly.
The reasons why the linearity coefficients between the instruments vary may be differences in the reflection from the reference specimen, differences in the optical adjustment, mechanical tolerances of the specimen holder, and so forth.
The reason for the nonlinear relationship between S.sub.s /S.sub.r and I.sub.s /I.sub.r may be a nonlinear detector characteristic curve, or scattered light, which depends on the reflection (transmission) of the specimen. Another reason for the nonlinearity may be multiple reflections at optical boundary layers because of great differences in the index of refraction. The nonlinear effects occur especially markedly at the point of contact with the specimen and are amplified even more if sapphire (index of refraction n=1.75) is used as window material instead of quartz (n=1.45), for instance. In many applications, however, sapphire windows are necessary, for instance because of their superior strength and resistance. It is therefore the object of the invention to improve the accuracy and replicability attainable for a number of spectral measurements of the same type.